1. Technical Field
The present invention relates to an acceleration sensor that detects the frequency change of a vibrating body when an acceleration is impressed.
2. Related Art
A known acceleration sensor includes a spiral spring, a resonator, and is formed with silicon having a vibrating mass suspended to a frame, where acceleration is detected based on a frequency change of a resonator, and where the spiral spring, the frame, and the vibrating mass are formed by structuring the silicon platelet. JP-A-6-43179 (page 3; FIG. 1) is an example of such related art.
Another known acceleration sensor includes: a cantilever, one end thereof anchored on a silicon wafer substrate, the other end thereof being a resilient free end; a piezoelectric element film formed on a surface of the cantilever; a piezoelectric resonator formed including metallic electrodes formed on both sides of the piezoelectric element film and a weight fixed to the free end of the cantilever. JP-A-2-248865 (page 2-3, FIGS. 3 and 9) is an example of such related art.
Another known acceleration sensor includes: a plated vibrating body; piezoelectric elements formed on the vibrating body facing each other; a supporting means supporting one end of the vibrating body; and a hole formed in a vicinity of one end of the vibrating body. Here, the vibrating body oscillates in longitudinal direction (i.e. longitudinal oscillation). The vibrating body and the piezoelectric element are bent due to the acceleration impressed in the oscillating direction of the vibrating body, and the acceleration sensor detects voltages generated by this bend in the piezoelectric element. JP-A-8-146033 (page 3, FIGS. 1 and 2) is an example of such related art.
Another known acceleration sensor includes: an inertial mass movable with acceleration; a support beam that supports the inertial mass and deforms when the inertial mass moves; and an resonance body disposed on the support beam; where the resonance body includes an excitation unit, a receiving unit that detects the excitation state, and a propagation unit that propagates the oscillation from the excitation unit to the receiving unit. Here, the magnitude of impressed acceleration is measured by detecting, with a usage of an input signal input into the excitation unit as well as an output signal output toward the receiving unit, the change in the oscillation state of the resonance body, deforming in accordance with the deformation of the support beam when the acceleration is impressed. JP-A-7-191052 (page 1-2, FIG. 1) is an example of related art.
The acceleration sensor according to JP-A-6-43179 detects the amount of frequency variability of the resonator caused by a bent of an accelerated spiral spring. Moreover, the vibrating mass is added in order to increase the detection sensitivity. Similarly in JP-A-2-248865, adding a weight in the end of the cantilever increases the detection sensitivity. Such vibrating mass and the weight are disposed in the direction to which acceleration is impressed. Therefore, as the energy necessary for oscillating the vibrating body increases, the impact strength may decrease.
Another problem involved here is the difficulty in making the acceleration sensor smaller.
The vibrating body according to JP-A-8-146033 oscillates in longitudinal oscillation. The amount of frequency variability in longitudinal oscillation is significantly smaller than that of a transversal vibration, resulting in a problem of difficulty in increasing detection sensitivity. Moreover, the supporting structure of the vibrating body becomes complex and therefore vibration loss may easily occur.
Still further, the structure of the vibrating body is such that a hole is formed in a vicinity of one end of the vibrating body, therefore easily causing a stress concentration, resulting in the decline of the impact strength.
The acceleration sensor according to JP-A-7-191052 has a structure to detect the deformation of the support beam using the resonance body adhered to the support beam. The support beam and the resonance body are made of different materials, and the thermal expansion coefficients thereof are different from one another. This causes the difference in the amount of deformation between the support beam and the resonance body when the temperature changes, and this deformation difference is output as frequency variability, resulting in a problem of poor temperature characteristics.
Further, propagation loss of a force provided by acceleration occurs at the section where the support beam and the resonance body are adhered together, and the long-term reliability is less likely to be ensured in such adhered section.
The precise detection of acceleration requires the positional precision of the resonance body in relation to the support beam. However, since the support beam and the resonance body are formed with different members, the positional precision is harder to achieve. This leads to a prediction of an increase in the manufacturing cost as well as of a difficulty in decreasing the size thereof.